Dept. of Electrical, Computer and Biomedical Engineering

Data acquisition from a temperature sensor

Data acquisition from a thermistor 2

Thermistors

A thermistor is a temperature transducer, typically featuring relatively fast response times, very good sensitivity, low cost but not so good linearity

Depending on whether the thermistor resistance decreases or increases with the temperature, we can talk about

NTC (negative temperature coefficient) thermistors, whose resistance decreases as the temperature increases

PTC (positive temperature coefficient) thermistors, whose resistance increases as the temperature increases

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NTC thermistors

They are generally built using a mixture of metal oxides (Ni, Mn, Fe, Cu, Co) with the property, similar to that of semiconductors, that the conductivity increases as the temperature is increased

As compared to PTC thermistors, they feature better linearity and a larger operating interval - their resistance-temperature characteristic is of the exponential kind

Data acquisition from a thermistor

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PTC thermistors

In PTC thermistors, the temperature coefficient increases very rapidly as soon as the so called Curie temperature is exceeded

Resistors whose resistance increases with the temperature - their operating principle is based on the particular properties of the material they are made of, sharply changing resistance with T (typically ceramic semiconductors, e.g. BaTiO3)

Data acquisition from a thermistor

They generally have larger temperature coefficient than NTC thermistors - on the other hand, their R-T characteristic is strongly non-linear and is provided by the manufacturer for predefined temperature intervals

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Main uses of thermistors

Thermistor Function PurposeNTC Temperature measurement •Heating systems

•Household electrical appliances•Industrial control

Thermal compensation •Industrial electronics•Consumer electronics•Data processing

PTC Thermal protection •Power supply circuits•Industrial electronics•Consumer electronics•Data processing

Overload protection •Power supply circuits•Telecommunication•Automotive•Industrial electronics•Consumer electronics•Data processing

Data acquisition from a thermistor

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Implement a system for data acquisition from a temperature sensor, in particular from an NTC thermistor. The system should include

Purpose of the experiment

The virtual instrument should take care of acquiring the temperature and representing the time evolution of the measured voltage and the instantaneous value of the temperature

a conditioning circuit for the signal coming from the sensor

a virtual instrument implemented in the LabView programming environment serving as an interface between the acquisition system and the user

Data acquisition from a thermistor

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Conditioning circuit

• R1=1.1 kΩ• C=100 nF• VCC+=+15 V• VCC-=-15 V• V1CC+=5 V • RT: KE 164

Data acquisition from a thermistor

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R-T characteristic for the thermistor

( ) ( )( )( )úû

ùêë

é×+

×=Þ×=

÷÷ø

öççè

æ-

0T

T0

0T1

T1β

0TT

TRTRlnTβ

TβTeTRTR 0

Thermistor resistance at temperature T

Thermistor resistance at the reference temperature T0=25 C

Temperature coefficient (about constant between 25 C and 100 C)

Data acquisition from a thermistor

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1OUT

OUTT

T

1T1

TOUT RvV1CC

vRV1CC

RR1

1V1CCRRRv ×

-=Þ×

+=×

+= +

++

( ) úúûù

êêë

é

×-×

×+

×=

+0TOUT

1OUT0

0

RvV1CCRvlnTβ

TβT

T-VOUT relationship

Measured voltage as a function of the thermistor resistance

Relationship between temperature and measured voltage

Data acquisition from a thermistor

10Data acquisition from a thermistor

TL081 JFET input OpAmp

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TL081 JFET input OpAmp

Data acquisition from a thermistor

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KE-164 NTC thermistor

Data acquisition from a thermistor

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Front panel

Waveform chart

Data acquisition from a thermistor

Number of samples to average

DEGREES

NTC resistance

Measured temperatureInstantaneous temperature [ C or K]

Voltage [V] NTC reference voltage (5 V nominally)

b coefficient (3850 K nominally)

14Data acquisition from a thermistor

DAQmx Create Channel.vi and DAQmx Read.vi

DAQmx Create Channel.vi provides the acquisition board with information about the type and range of the signals to be acquired and about the input channel

DAQmx Read.vi samples the signal from the specified channel and yield the measured value

input channel (Dev2/a1)

expected limits for the signal to be acquired

Measurement I/O -> DAQmx Data Acquisition -> DAQmx Create Channel.vi

Measurement I/O -> DAQmx Data Acquisition -> DAQmx Read.vi

15Data acquisition from a thermistor

While loop

Needed for continuous acquisition of the signal coming from the conditioning circuit (you can find it in the Structures menu from the Functions palette ) – a “stop” button should be included in the virtual instrument to stop the acquisition

loop condition

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Formula node

The Formula Node structure can be used to calculate the relationship between the temperature T and the measured voltage VOUT based on the resistance-temperature relationship (you can find it in the Structures menu from the Functions palette)

independent variable

dependent variable

Data acquisition from a thermistor

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Waveform chart

You can use a waveform chart for a graphical representation of the acquired data (‘Graph’ menu of the Controls palette, from the front panel window) – the acquired sample can be directly fed to the waveform chart

Data acquisition from a thermistor

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For cycle

number of cycles

iteration number

We can use a for cycle to reduce the effects of zero average disturbances, therefore improving the measurement accuracy

Instead of representing (in the graph or in the numeric indicator) each individual acquired sample of the signal, we can represent the average value of N samples – the speed at which the measurement result is represented on the graph will decrease by a factor of N

Data acquisition from a thermistor